The present invention relates to a stock feed system for a multi-layer headbox and a method in the operation of a multi-layer headbox.
In the prior art, systems for the operation of multi-layer headboxes are known in which there are separate fresh stocks for the surface layers of a pulp flow discharged from the headbox and for the middle layers thereof. Thus, in the prior art systems, there have been at least double fresh-stock systems for the formation of the different pulp layers. In the prior art equipment, the stocks introduced along at least two separate fresh-stock lines have been processed in vortex cleaning and in deaeration tanks, and feeds of fillers or starch complying with the required paper grade have been passed into the at least two fresh-stock lines.
With the prior art technology, a paper machine into whose headbox stock is fed at different consistencies requires a complete short circulation which comprises more than one stock systems. This is also the case when the stock in the different layers is in the other respects the same, but the consistencies are different.
Increased consistency of layers may be necessary, for example, in order that an improved purity of the layers can be achieved. When the middle layer is thicker than the surface layers, water of the middle layer is drained through the surface layers to a relatively limited extent. In such a case, particles of the middle layer are not carried along with the water into the surface layer, and the purity of the layers remains good.
A very thick middle layer is highly flocculated, in which case it has a high bulk. Then, the structure of the paper results in a bulky paper of good bending stiffness, in which the formation of the middle layer is poor but the formation of the surface layers is good, which secures good printing properties.